refactor: self-document error flag with a constant

The previous implementation required the caller
to have knowledge of its platform's `getuid` error
code. Although this had the benefit of replacing a
hard-coded error value with a descriptive variable,
the attempt at future proofing `get_user_id` was
flagged as a poor design decision by Nicoddemus:
to future proof the function, `get_user_id` should
determine the error flag internally.

However, future-proofing was not the reason
the issue was opened. The issue was opened to
resolve an `[attr-defined]` error on Windows
platforms caused by *mypy*'s handling of
code reachability through platform checks.

Over the course of the open issue, additional housekeeping
tasks were also undertaken:
- adding a comment that links to an explanation of
*mypy*'s platform and version checking requirements
- adding a variable name and comment to clarify
the reason for the conditional return `return uid if uid . . .`

src/_pytest/compat.py

@@ -313,14 +313,10 @@ def safe_isclass(obj: object) -> bool:
         return False
 
 
-def get_user_id(*, UNRELIABLE: int = -1) -> int | None:
+def get_user_id() -> int | None:
-    """Return the current user id, or None if we cannot get it reliably on
+    """Return the current process's real user id or None if it cannot be
-    the current platform.
+    retrieved reliably.
 
-    :param UNRELIABLE:
-        The platform-specific constant which indicates that the retrieved uid
-        is unreliable. The default value, -1, is a common unreliability
-        indicator on UNIX-like systems.
     :return: The user id or None
     """
     # mypy follows the version and platform checking expectation of PEP 484:
@@ -331,6 +327,9 @@ def get_user_id(*, UNRELIABLE: int = -1) -> int | None:
         # Emscripten has a return 0 stub.
         return None
     else:
+        # On other platforms, a return value of -1 is assumed to indicate that
+        # the current process's real user id cannot be retrieved reliably.
+        UNRELIABLE = -1
         uid = os.getuid()
         return uid if uid != UNRELIABLE else None